FIELD OF THE INVENTION
The present invention generally relates to metal foils and their manufacture, and more particularly relates to an improved method and composite for making supported thin copper foil.
Copper foil has been used for many years in the manufacture of printed circuits. The raw material for printed circuits usually is a laminate of copper foil and a thermosetting plastic, such as a phenolic or epoxy resin, which is often supported by paper, glass, cloth, or fibers. The plastic is usually applied in the uncured resinous state to the copper foil and is then heat and pressure cured to cause it to firmly bond to the foil. The finished laminate can have one or both exterior surfaces clad in this way with copper foil, whose thickness, by weight, is usually about 1 to 3 oz. per sq. ft. The various methods of making printed circuits from such laminates are well known to those skilled in the art.
In recent years it has been found desirable to make laminates for printed circuits using copper foil that is much thinner than formerly. This is dictated by a new requirement that for certain applications conducting elements be much smaller in width than previously and be spaced much closer together to facilitate circuit miniaturization. There is also a need to reduce the amount of copper removed by etching (to form the circuit conductor pattern) so as to reduce the cost of etchant and decrease etchant disposal problems.
Copper foils in thicknesses down to about 0.5 oz. per sq. ft. are made by electroplating the copper foil onto the surface of a slowly rotating steel drum, whose surface is specially prepared to prevent the copper layer from strongly adhering thereto. By controlling the speed of rotation of the drum and the electroplating conditions, the copper foil can be electroformed to a desired thickness. It is then peeled from the drum surface and wound onto a roll for further treatment, storage, or shipment. The surface of the foil formed away from the drum is considerably rougher than is the drum surface and is usually further treated to impart microscopic projections to it, so as to enhance its bondability to plastic during the subsequent lamination step.
When copper foil is made in a thickness of about 0.5 oz, per sq. ft. or less, however, it becomes very difficult to handle. Thus, when it is cut into sheets and placed in the laminating press, it often wrinkles and tears unless a carrier is used to support it or unless extreme care is exercised. For copper foils of thicknesses much less than 0.5 oz. per sq. ft., the use of a thin carrier material such as a plastic film or a metal foil is often mandatory.
Thus, in current practice, the desired thin copper foil layer is electroformed on the surface of the foil or film carrier and may also be treated while being supported there in order to enhance its subsequent bondability to the permanent plastic substrate in the laminating step.
Two general types of carriers used for such purposes are metals and plastics. Thin copper foil layers usually adhere only lightly to plastic carriers but usually adhere strongly to metal carriers. Separation of the foil from a plastic carrier is usually accomplished merely by peeling away the carrier, since the foil is usually only lightly attached thereto. But when the copper foil and carrier are strongly bonded together, that is, when the carrier is metal, then their separation must usually be attained by drastic means, such as by chemically dissolving the metal carrier away from the remaining thin copper foil.
After the copper foil is formed and treated on whatever carrier is employed, such foil and carrier are cut to the size necessary for the completed laminate. The carrier-supported foil is then assembled with the uncured usually reinforced plastic in a press, whereupon heat and pressure, for example, 200-400 deg. F. and 50-500 p.s.i., are applied to permanently bond the treated thin copper foil surface to the reinforced plastic base. Since the carrier is still attached to the other surface of the thin copper foil, it must then be removed either by peeling it away or by dissolving it off with chemicals, as described above.
In the case where the carrier is a plastic film, it must first be chemically prepared to receive a very thin metal layer by electroless desposition or by vacuum deposition and then the thin copper foil layer can be electroplated to the desired final thickness on the thin metal layer. The procedures required to chemically prepare the plastic for the initial metal layer, however, are intricate, time consuming, and require the use of expensive chemicals. Consequently the use of plastic film as the carrier material is expensive and thin copper foil produced on such a carrier is quite expensive. The preparation of plastic film by vacuum deposition of metal thereon is also expensive and reliable methods for metallizing plastics without pinholes of various sizes have not yet been developed.
Because of these deficiencies, metal foil has been considered for use as the carrier. Thus, it is electrically conductive, so that the metallizing step is averted. The particular foils that have been commercially used for this purpose have largely been copper and aluminum. Before the thin copper foil layer can be deposited over the copper carrier, however, the surface of the copper carrier must be prepared to prevent too strong an adhesion from occurring.
Various complicated procedures have been employed for this purpose, and have met with limited success. Their continued use has been motivated by the fact that it is quite expensive and difficult to dissolve the thicker copper carrier from the thin copper foil. A further problem, however, is that the copper carriers themselves are 2 mils thick, are made by electrodeposition and they are quite expensive. Like the plastic carriers, the high cost of thin copper foil produced on copper foil carriers has therefore severely restricted commercial applications.
From the standpoint of lowest cost, the most suitable materials for use as carriers for the thin copper foil would be such inexpensive metal foils such as aluminum, brass, steel, and stainless steel, in thicknesses of from 1 mil up to 10 mils, depending upon the hardness or temper of the foil. Until now, however, there has been no way to prepare these metal foils to accept a thin, nonporous copper foil layer that is lightly bonded to the carrier surface.
The use of aluminum foil as a carrier has been explored and two methods have been proposed in the past to enable the ue of the aluminum as a carrier. The first method employs copper plating directly over the inherent oxide that is always present on unprotected aluminum, but first treats the aluminum surface in an attempt to produce a uniform controlled oxide layer. This is very difficult to do, however, cannot be reliably reproduced and thus attempts usually result in the production of porous copper foil layers with widely varying adhesion to the carrier. Such foils do not separate properly, and thus cannot be recovered intact.
The second method uses the procedure called "zincating", in which the oxide of the aluminum is removed and replaced by a thin layer of zinc. However, when the thin copper foil is deposited over the zinc the adhesion between the copper foil and the zinc layer is quite strong. The second method has permitted aluminum foil to be used as a carrier, but only with the eventual separation of the thin copper foil from the zincated aluminum carrier by the expensive procedure of dissolving away the aluminum, usually in a hot caustic solution.
Therefore, there still is a substantial need to provide a simple economical method for making thin pore-free copper foil.